The present invention relates generally to a new automated laser welding system configured to produce, for example, an improved welded work piece, such as an automotive body panel, and a system and method for the manufacture thereof that includes an improved laser welder and a visual weld inspection device. The invention also relates to a method for performing an automated quality control inspection of a laser weld.
In the past, welded work pieces such as body panels for use in the automotive vehicle industry were made by stamping or drawing the panel from either a single blank of a ductile sheet metal material, including steel, or from a plurality of such blanks that were previously welded together. Either type of welded work piece or body panel usually required the addition of stiffeners and pads welded to sections of the panel to improve its structural rigidity. The added stiffeners and pads were also needed to increase the thickness of the work piece in predetermined locations so that various structural and fastening assemblies could be fastened and welded to the panel without damage during the fastening or welding process. The addition of the stiffeners and pads increased the weight of the work piece and also increased the total manufacturing time needed to fabricate the work piece. The work pieces were often formed, drawn, or stamped into a final shape to have a three-dimensional shape corresponding to the overall design of the automotive vehicle.
As a result of the number of manufacturers in the field, the automotive vehicle industry is very competitive with respect to, among other things, quality, raw material costs, and manufacturing times required to completely fabricate and assemble a vehicle. To remain competitive, manufacturers have continuously expended enormous resources to contain, if not reduce, material costs by reducing part weight, part count, and manufacturing time while maintaining the needed high degree of quality. A considerable amount of such resources have been directed to improving and automating routine tasks such as the fastening together of various work pieces and vehicle parts such as, for example, body panels for fenders, quarter panels, trunk lids, engine compartment hoods, vehicle doors, and other various components.
Previously, multi-part sheet metal blanks have been welded together into a single work piece before being stamped into a final shape. These blanks were prepared by a variety of fastening techniques including chemical, arc, and CO2 laser welding, riveting, bolting, cold forming, and similar methods. Of particular interest in recent years is the use of more efficient laser welding using CO2 lasers in automated, numerically controlled manufacturing processes. Such laser welding can be accomplished for joining together sheet metal blanks at a common seam by means of, for example, a lap weld, or a butt weld. Butt welds are often preferred because only a single seam needs to be welded in contrast to lap joint which usually require that two seams be welded.
Many problems have been associated with the use of CO2 lasers including the requirement that less than optimum welding speeds must be used because of the poor absorption by steel work pieces of the energy produced by the CO2 laser. Also, laser welded joints can be plagued with problems despite the use of an appropriate weld speed if a manufacturer does not carefully prepare the work pieces or is otherwise not attentive to the intricacies and pitfalls of laser welding processes. Problems are even more prevalent when the blanks to be welded together are of dissimilar thickness. Such problems include, for example, mismatch between the welded parts along the joint on at least one exterior surface, poor weld bead dimensions or hardness, cracks, poor weld bead continuity across the length of the weld, and pinholes formed in the weld bead. Many of these welding problems are difficult to avoid and even more difficult to detect. More often than not, detection of such problems can only be accomplished by a slow and tedious visual inspection. Further, some of these problems, such as cracks, weld spatter, and pinholes, can only be detected through destructive testing such as by tension and shear tests, micrographic cross-sectional analysis, etch and penetrant dye inspections, and formability testing to ensure the welded blanks of the work piece can be drawn or stamped without failure anywhere along the welded joint.
These problems are especially apparent when steel work pieces, such as welded components for an automotive body or door panel, are to be butt welded together for form a larger, single work piece or door panel blank that can be later stamped or drawn into a shaped panel ready for painting and attachment to the vehicle. In many cases such welds are straight line weldments that could be completed faster if an improved laser welding technique were available. Additionally, it would be desirable to have an automated manufacturing assembly line wherein multiple work pieces could be automatically introduced to the laser welding apparatus to minimize the risk of injuries to workers from reflected laser energy. Further, such welding manufacturing processes could be made more efficient if a technique existed to speed up the post-weld inspection process.
There have been attempts to develop a viable method for laser welding inspection. U.S. Pat. No. 5,607,605 discloses such a method, which utilizes a CCD (Charge Coupled Device) camera to capture an image of the plasma generated when a laser beam contacts an object to be welded. The image is then sent to an image processing device, which measures a selected particular feature of the plasma cloud. The measurement is further transferred to a distinction device, which compares the measurement with a reference value to determine if the laser welding condition, and thus the weld, is acceptable.
Electro-optical detection of laser welding conditions has also been employed as an inspection method. U.S. Pat. No. 5,272,312 recites a method for the inspection of a laser weld, wherein the area of the material in contact with the laser beam, referred to as the laser processing spot, is projected onto at least one photodetector such as photodiode which detects the amount of liquid material ejected from the weld pool during the welding process. The signal from the photodiode can be converted into an electrical signal, which may then be sent to a processing unit for determination of the size and location of voids or pores in the weld seam. In one embodiment, this reference discloses the detection of ultraviolet radiation present in the plasma cloud.
Laser welding generates particular signals which may be monitored to determine the quality of a weld. U.S. Pat. No. 5,681,490 discloses that sensors such as photodiodes, phototransistors, photo darlingtons, pyroelectric detectors, microphones, and infrared and thermal detectors can be positioned to monitor various stages of the welding process. Such sensors may be utilized to monitor light, sound, gas, smoke, temperature, etc. The signals generated by these sensors may then be analyzed by a computer to predict the weld quality.
None of the prior art, however, discloses an apparatus or method utilizing direct inspection of the weld bead to determine the quality of a laser weld. The prior art methods generally depend upon the use of unstable process indicators to ascertain the condition of the weld, often requiring the monitoring and analysis of a multitude of signals to reach a conclusion regarding weld quality.
The automotive industry is in need of a laser welded work piece that contains fewer parts, has an optimally minimized weight, and that is produced through the use of an automated, laser welding manufacturing process. The welded work piece produced in accordance with the present invention, and the system and method for its manufacture, overcomes the deficiencies of the presently known methods for automated laser welding and inspection of welded work pieces.
In general, the present invention is directed to an improved laser welded work piece and an automated laser welding and visual inspection system and method configured to manufacture the work piece. The welded work piece incorporates a minimized gap that is designed to improve the structural properties of the laser weld. The new automated manufacturing system includes a robotically automated production line configured to prepare blank work pieces for welding by precision shearing at least one edge, and to precisely align the blanks and laser weld them together using a single or dual cell, high-speed, high-power laser. During welding, the laser weld is concurrently inspected by a visual inspection device to determine whether the welded work piece should be accepted or rejected. The operator can continuously supply palletized raw materials, such as pallets or skids of sheet metal blanks, to the production line without stopping or interrupting the automated production line. After welding, the system robotically sorts and re-palletizes the finished, welded work piece onto accepted work piece skids or onto rejected work piece skids. The operator can remove the accepted and rejected work pieces from the production line without stopping or interrupting the continuously running line.
The Welded Work Piece
The invention includes a welded work piece for use in manufacturing an automotive vehicle that incorporates a first blank of a steel sheet stock with a first thickness and having at least one first precision sheared edge, and a second blank formed from a steel sheet stock material of a second thickness having at least one second precision sheared edge. The first and second precision sheared edges are produced in the respective first and second blanks to form a minimized gap between the edges before welding. The edges are laser welded using the apparatus disclosed herein, to form a beaded seam that permanently joins the respective first and second blanks. In a method for manufacturing the improved welded work piece, first and second blanks of a sheet stock steel are selected to be of similar or dissimilar respective thickness and respective precision sheared edges. The edges are positioned on a flat welding surface and tightly compressed together in an abutting relationship to form a minimized gap between the edges. The edges are then laser welded together to form a beaded seam that permanently joins the blanks together to form the welded work piece.
There is thus disclosed a welded work piece for use in manufacturing an automotive vehicle, comprising first and second sheet metal blanks, each formed with at least one precision sheared edge and having similar or dissimilar thickness. The blanks form a minimized gap when the respective at least one precision sheared edges are positioned in an abutting relationship A continuous wave laser butt welded seam fixedly joins the blanks together along at least one of the respective precision sheared edges.
The System
Another aspect of the present invention is directed to a system for manufacturing a welded work piece. The system includes at least one articulating arm feeder robot, configured to retrieve at least one sheet metal blank from a plurality of such blanks, from at least one of a plurality of feeder skids containing palletized sheet metal blanks. The arm is adapted to transport the individual blanks, one at a time, from the skid to a load position on a magnetic conveyor. Each of the blanks are formed with at least one joining edge. In applications where blanks of dissimilar thickness or other dimensions are used, the blanks may either be stacked alternately on a single skid, or a second articulating arm feeder robot may be employed to retrieve a dissimilar blank from a second plurality of such blanks that are palletized on a second plurality of skids. The second robot arm operates cooperatively to feed the second, dissimilar blank onto the magnetic conveyer.
The magnetic conveyor of the system is adapted to receive from the feeder robot or robots at least two blanks. The conveyer is configured with blank locator devices adapted to precisely position them on the substantially flat conveyor bed. The blanks are proximally pre-positioned so each of the respective joining edges are substantially parallel. The magnetic conveyor is further configured to releasably restrain the positioned blanks into place and to move the blanks from the load position to a shear position.
The system also incorporates a precision shear device positioned about the shearing position of the magnetic conveyer. The shear device is configured with at least one upper stamping die that cooperates with at least one lower stamping platen to precisely shear at least one of the respective joining edges of each of the blanks. After shearing, the blanks are moved by the magnetic conveyer onto an idle station that temporarily stores the sheared blanks until they can be welded. The blanks are then conveyed by a second conveyer to a welding gantry located at the other end of the idle station.
The second conveyer moves the sheared blanks onto a laser weld bed of the welding gantry. The gantry includes a clamping and positioning assembly operative to releasably register and press the respective joining edges of the blanks flat against the weld bed and tightly together with the edges in an abutting relationship to form a minimized gap. The clamping mechanism is configured with a clamp assembly having multiple bars that clamp down on each blank to firmly press them against the laser weld bed. The positioning assembly includes a plurality of locator assemblies that push against one or more of the non-joining edges of each blank to precisely locate the blanks so that the precision sheared edges are tightly pressed together. When so pressed together, the edges form a minimized gap or seam therebetween.
The system also includes a laser welder movably attached to the welding gantry. The laser welder may be configured with a weld head powered by a remote laser power unit. The weld head moves along the gantry and, when energized, projects a laser beam incident to and focused upon the gap or seam of the blanks to form a weld bead seam. The system also comprises a laser weld inspection device that is adapted to move along either in conjunction with or independently of the laser weld head to inspect to the weld bead seam. Once welded, an exit conveyor operates to remove the welded work piece from the laser weld bed. An articulating arm exit robot is also included that is configured to move the work piece from the exit conveyor to an exit station. If the inspection revealed that the weld was acceptable, the exit robot moves the welded work piece to one of a plurality of skids for work pieces that have passed the inspection. Otherwise, if the inspection revealed that the weld bead seam was not acceptable, the exit robot moves the defective welded work piece to one of a possible plurality of reject skids.
There is thus disclosed a system for manufacturing a welded work piece, comprising at least one articulating arm feeder robot, configured to retrieve at least one blank from at least one of a plurality of feeder skids of palletized sheet metal blanks. Each blank is formed with at least one joining edge, and the articulating arm feeder robot is adapted to transport the blank to a load position on a magnetic conveyor. The magnetic conveyor is adapted to receive from the feeder robot or robots at least two of the plurality of blanks and to precisely position them on a conveyor bed. The blanks are proximally pre-positioned so each of the respective joining edges are substantially parallel, and the magnetic conveyor is further configured to releasably restrain the positioned blanks into place and to move them from the load position to a shear position. The system further comprises a precision shear device, positioned about the shearing position of the magnetic conveyer, and configured with at least one upper stamping die that cooperates with at least one lower stamping platen to precisely shear at least one of the respective joining edges. There is also a welding gantry, spaced apart from the precision shear device and configured with a second conveyor having a laser weld bed and connected to the magnetic conveyer with an idle station therebetween. The second conveyer is configured to slidably receive the sheared blanks from the idle station and to move them onto the laser weld bed. The system also utilizes a clamping and positioning assembly operative to releasably register and press the respective joining edges of the blanks flat against the weld bed and tightly together in an abutting relationship to form a minimized gap. A laser welder is movably attached to the welding gantry, and has a weld head powered by a remote laser power unit to project a laser beam incident to and focused upon the gap for welding the blanks along the gap to form a weld bead seam. A laser weld inspection device is slidably coupled to the welding gantry and operative to inspect the weld bead. After inspection, an exit conveyor coupled to the second conveyor, removes the welded work piece from the laser weld bed. An articulating arm exit robot moves the work piece from the exit conveyor to an exit station, which is selected from the group of one of a plurality of accepted work piece skids or a rejected work piece skid.
The system further includes a light curtain system that is configured to surround each of the plurality of the feeder and exit station skids to allow removal and replacement of empty feeder and full exit skids without the need to interrupt the operating manufacturing system. If an operator approaches any of the skids for removal and replacement, the light curtains signal the robots either directly or indirectly. In response, each of the robots is directed to another of the plurality of skids for purposes of retrieving unwelded blanks or outputting welded work pieces during the period of time that the light curtain is activated. Similarly, each of the skids or a skid holder unit incorporates a sensor that either signals that the skid is empty or full. If either of these conditions occurs, the robot is directed to use another of the plurality of skids.
There is further disclosed a method for manufacturing a welded work piece comprising the steps of: shearing a precision edge on a respective joining edge of a plurality of sheet metal blanks, using a precision shear device configured with at least one upper stamping die that cooperates with at least one lower stamping platen to perform the shearing operation; moving the plurality of precision sheared blanks together on a conveyor from the precision shear device to a laser weld bed of a welding gantry; precisely locating the blanks to register the precision sheared edges in a compressed, abutting relationship; clamping the respective joining edges of the blanks flat against the weld bed and tightly together in an abutting relationship to form a minimized gap; and laser welding the edges to form a beaded seam and to permanently join the blanks together.
The Laser Welder
In yet another aspect of the present invention, a single or multi-celled laser welder is described. The laser welder incorporates at least one laser weld head that is configured to movably project at least one laser beam onto a plurality of work pieces to weld them together. As described above, the work pieces are positioned so that the edges are tightly pressed together in an abutting relationship to form a seam or gap. The work pieces are welded together with a laser weld head that projects the laser beam incident to the gap with a compound angle. The compound angle is measured relative to the vertical direction substantially normal to the substantially flat sheet metal work pieces. A leading angle component of the compound angle is substantially in the direction of movement of the laser weld beam as the weld head moves across the blanks during welding. A leaning component of the compound angle is orthogonal to the leading angle and is substantially in the direction normal to the blanks and the gap and it leans to one side towards one of the blanks away from the vertical direction.
Thus, there is disclosed a laser welder for welding a plurality of work pieces, comprising a laser weld head configured to movably project a laser beam onto a minimized gap formed between a plurality of adjacent, substantially flat work pieces formed with respective precision sheared edges. The edges are positioned in an abutting relationship, and the laser weld head is operative to weld the edges by forming a weld bead seam between the edges. The laser welder further comprises a laser beam incident on the gap with a compound angle. The compound angle is measured relative to the vertical direction substantially normal to the work pieces, and includes a leading angle component substantially in the direction of movement of the laser weld beam, and a leaning component substantially in the direction normal to the gap and leaning towards one of the blanks away from the vertical direction.
There is further disclosed a multi-celled laser welder comprising a plurality of laser weld heads, each configured to movably project a laser beam onto a plurality of minimized gaps formed between a plurality of adjacent, substantially flat work pieces formed with respective precision sheared edges. The edges are positioned in an abutting relationship and the laser weld heads are operative to weld the edges by forming a weld bead seam between the edges. The laser beams are incident on the gaps with a compound angle. The compound angle is measured relative to the vertical direction substantially normal to the work pieces, and includes a leading angle component substantially in the direction of movement of the laser beams and a leaning component substantially in the direction normal to the gaps and leaning towards one of the blanks away from the vertical direction.
The Inspection System
The present invention is also directed to a specially designed vision system configured to inspect a laser weld bead in real time. When the focal point of a laser beam contacts a work piece, it generates intense heat which forms a molten weld pool. As the laser beam traverses the work piece, the weld pool left behind quickly cools to form a weld bead. A visual sensor, such as a CCD (Charge Coupled Device) or video camera follows the laser welding head to view the weld bead. Although in a preferred embodiment of the invention a visual sensor is affixed to and travels with the laser welding head, it should be realized that the visual sensor could also be detached and independently propelled. An image of the weld bead is captured by the visual sensor at a predetermined interval based on the velocity of the laser welding head and other factors. The image from the visual sensor is sent to an image processing board, which in conjunction with a coprocessor board, computer and the system software, compare the image to a list of predefined, preferred tolerances, which correlate with several established characteristics of the weld bead considered to be acceptable. If it is determined that the selected characteristics of the weld bead image are within the specified predefined tolerance limits, a signal is generated classifying the weld as acceptable. If it is determined that the image is outside the specified predefined tolerance limits, a signal is generated classifying the weld as defective.
Thus, there is disclosed a laser welding inspection system comprising a laser welding device, an image capturing device for capturing the image of a laser weld bead, and an image processing device in electronic communication with the image capturing device, for measuring at least one dimension of the laser weld bead image captured by the image capturing device. The system further comprises a distinction device, in electronic communication with the image processing device, for comparing the value of the at least one dimension of the laser weld bead image measured by the image processing device with a reference value, to determine the quality of the laser weld.
There is further disclosed a method of inspecting a laser weld, the method comprising capturing an image of a weld bead, measuring at least one dimension of the laser weld bead image, and comparing the value of the dimension of the laser weld bead image with a reference value to determine the quality of the laser weld.
Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the features of the present invention.